1. Field of the Invention
The present invention relates to a profile measuring instrument.
2. Description of Related Art
A known profile measuring instrument includes: a probe having a measurement unit for measuring an object; a movement mechanism for moving the probe; and a controller for controlling the movement mechanism, the measurement unit being moved along a surface of the object to measure a profile of the object (see, for instance, Patent Literature 1: JP-A-55-154408 and Patent Literature 2: JP-A-07-324928).
A coordinate measuring machine according to the Patent Literature 1 uses acceleration of respective driven components or acceleration obtained by a drive motor in order to compensate measurement errors caused by a deflection of a movement mechanism on account of inertial force during the scanning movement of the probe, i.e. dynamic deflection (dynamic Abbe's error) caused by a deformation of the movement mechanism.
The CMM (Coordinate Measuring Machine) disclosed in the Patent Literature 2 calculates a compensation value representing deflection characteristics of the movement mechanism as a function of the probe position and acceleration in order to compensate the aforementioned dynamic Abbe's error.
FIG. 12 is an illustration showing a model of Abbe's error to be compensated by a conventional profile measuring instrument.
Specifically, the CMM disclosed in the Patent Literature 2 models the Abbe's error as a spring SP connecting a probe position Ps (xs, ys, zs) and a compensated probe position Pc (xc, yc, zc) as shown in FIG. 12. A spring constant K of the spring SP represents the deflection characteristics of the movement mechanism, which are dependent on the probe position Ps.
Accordingly, for instance, a relationship between the probe position xs and the compensated probe position xc in X-axis direction can be expressed by the following formula (1) based on the spring constant K and a force F applied on the spring SRxc=xs−K·F  (1)
Further, since the force F is caused on account of the inertial force during the scanning movement of the probe, the force F is proportional to acceleration a of the probe. In other words, the relationship between the probe position xs and the compensated probe position xc can be expressed by the following formula (2), where Ka represents a proportion constant between the force F and the acceleration a of the probe.xc=xs−K·Ka·a  (2)
As described above, the CMM disclosed in the Patent Literature 2 can compensate the Abbe's error based on a function of the probe position Ps and the acceleration a.
However, since the CMM disclosed in the Patent Literature 2 compensates the Abbe's error based on the function of the probe position Ps and the acceleration a, the Abbe's error cannot be appropriately compensated when the model of the Abbe's error has to be expressed by a high-order transfer function.
Specifically, since the CMM disclosed in the Patent Literature 2 compensates the Abbe's error based on the function of the acceleration a, if one tries to transform the model of the Abbe's error into a function of, for instance, frequency (e.g. a function of rotation frequency of the scanning movement of the probe according to the present invention, which will be described below), only a second-order term of the function can be expressed. Further, the model of the Patent Literature 2 cannot be directly transformed into a transfer function. In the Patent Literature 2, instead of a given mass M, the force F is obtained by acceleration (movement condition) of the mass M to be used for compensation.